It has become increasingly important in recent years to be able to detect and measure very low concentrations of boron in deionized water with a high degree of accuracy and high reproducibility of results. For example, in certain industrial applications, such as semiconductor manufacture, even very low levels of boron in deionized water used in manufacturing can significantly and adversely affect the quality and performance of the resulting products.
Large amounts of ultrapure water are required in processes to manufacture semiconductors. Boron is one of the contaminates that must be removed to very low concentrations. Boron is a semiconductor p-type dopant used in manufacture of solid state electronics and functions as a principal charge carrier in a crystal of silicon. Accordingly, boron must not be added inadvertently during the manufacturing process. S. Malhotra et al. reported in “Correlation of Boron Breakthrough versus Resistivity and Dissolved Silica in RO/DI System” (Ultrapure Water, May/June 1996. 13(4): p. 22-26) that boron was the first ion to breakthrough the ion exchange resin beds when they switched to thin-film-composite (TFC) reverse osmosis membranes. The introduction of TFC reverse osmosis (RO) membranes (to replace cellulose acetate RO membranes) was very effective in reducing the silica passage of the RO apparatus. The reduction in boron passage was not as great, however. The first ions to leak from the mixed ion exchange resin beds that follow the usual RO pretreatment are weakly ionized compounds such as silica and boron. With TFC RO membranes, silica passage is much less than boron passage. Boron is therefore often the first ion to breakthrough mixed ion exchange resin beds in water purification systems that use RO TFC pretreatment. This is especially true if the feedwater contains high levels of boron. Adsorption of borate ion on anion exchange resins is the most common method to remove boron from ultrapure water. As the resin begins to become exhausted, however, borate is one of the first ions to leak through. Such borate leakage can rapidly exceed acceptable concentration levels.
Thus, there is a need in semiconductor manufacture and many other applications to be able to monitor very low levels of boron in water quickly, accurately, inexpensively, and while a deionized water stream is on-line. Obtaining reliable measurement of very low levels of boron in water also requires methods and apparatus that eliminate or at least minimize the many possible sources of small errors in conventional approaches, for example, unnecessary possible contamination of samples, sensitivity of readings to flow rate variations, and small changes in aqueous conductivity caused by polyol added to enhance ionization of boric acid. In the past, such small sources of errors have largely been disregarded as insignificant relative to the relatively high concentrations of boron being measured. It will be apparent, however, that as it becomes necessary to measure boron at increasingly lower concentrations, even very small measurement errors inherent in prior art measurement methods become increasingly significant and lead to large measurement distortions on a percentage basis.
It is generally known in the art to form conductive polyol-boron complexes and to utilize conductometric detection techniques. These prior art conductometric polyol-boron complex methods, as well as prior art colorimetric methods, however, are not sensitive enough to detect very low concentrations of boron in purified water used in semiconductor manufacturing. Also, to measure very low levels of boron in water using prior art methods typically requires some form of pre-concentration of boron in the water sample to achieve sensitive enough detection. These added steps introduce additional errors and complexity to the measurement process. Some of these prior art processes for boron detection and their limitations and deficiencies are discussed further below.
One prior art approach to boron measurement is ICP-MS. ICP-MS detection of boron with a pre-concentration step is currently the most sensitive measurement method reported in the literature. The limits of detection are reported to be about 0.005 ppb as B. Without pre-concentration, however, the limit of detection using ICP-MS is much higher, about 0.050 ppb as B. The cost of ICP-MS apparatus is very high, and this fact prevents its common use on-line to measure boron concentrations in real time.
Dionex Corporation has developed an ion chromatographic method to measure boron concentrations. This method uses a styrene-based resin with polyhydroxyl functional groups attached for pre-concentration. The resin polyhydroxyl group-boron complex formation constant is smaller than the boron-mannitol complex formation constant. This allows the resin to collect boron from a water sample; and, after such collection is complete, boron can be removed from the resin using 100 mM mannitol and 2 mM H2SO4 eluent. The concentrated boron is carried to a resin-packed ion separation column. The separation column resin has an anionic charge. This allows ion exclusion separation of the borate-mannitol anion. Conductometric detection is then used to measure the borate-mannitol complex. The limit of boron detection with the pre-concentration column was reported as 0.050 ppb as B. This system is not well suited for on-line measurement of boron in water, however, due to its complexity. This system of boron analysis was reported at the 1998 SPWCC by B. Newton (SPWCC, pp. 197-216, 1998).
P. Cohen et al. (U.S. Pat. No. 3,468,764) described another design for a boron-in-water analysis method and apparatus. Such device introduces boron-containing water and, periodically, a known boron standard, to a bed of compressed mannitol spheres, and then measures the conductivity of the resulting boron-mannitol complex. The conductivity difference between the water sample and the boron standard is proportional to the concentration of boron in the sample. This device is used in boron concentration ranges commonly found in the nuclear power industry. The detector response is reported to be linear at a first slope over a concentration range of 800 to 3200 ppm and also linear over a concentration range of 0-800 ppm but with a second very different slope. Boron has a high radiation cross section to neutrons and is thus used to control nuclear reactors. There are a number of differences between the Cohen et al. '764 patent and the present invention.
The Cohen et al. device does not take account of the necessity to measure the conductivity of polyol/boron-free water to obtain accurate low level boron measurements. This oversight introduces unacceptable errors into measurement of very low boron concentrations.
The Cohen et al. '764 patent does not compare the conductivity of a polyol/boron-containing sample with the conductivity of a polyol/boron-free sample, a feature that is a key aspect of the present invention.
The Cohen et al. device adds mannitol by flowing the sample or boron standard over a bed of mannitol spheres. By contrast, in a preferred embodiment the present invention injects a small volume of a concentrated, even saturated, polyol solution into a micro stream of a boron-containing sample or a boron-free sample. The Cohen et al. method of polyol addition is sensitive to sample flow rate and to the instant surface area of the mannitol bed. If residence time is too short, the mannitol concentration will change. If flows could be controlled so that mannitol concentration approached saturation, mannitol concentration would then be stable. Such a modification, however, is neither described by Cohen et al. nor compatible with the present design. The Cohen et al. design is in addition generally wasteful of mannitol.
Finally, Cohen et al. do not teach the deionization of polyol solution to remove ions that could contaminate the conductivity of the polyol before its addition to the water sample, thereby again introducing inconsistencies and inaccuracies into the boron measurements.
Ikuo Yabe (U.S. Pat. No. 4,204,259) teaches a device that can be used on the primary cooling water in a pressurized water atomic power plant, and, therefore, is generally concerned with measuring relatively high concentrations of boron. In such patent, a mannitol solution and a boron-containing water sample (or a boron standard) are blended together. The conductivity of such boron-containing water sample (or such boron standard) is measured in a first conductivity cell before mannitol is added. The mannitol/boron-containing sample is passed through a thermal correction device and then into a second conductivity measurement cell. This device measures lithium (Li) with the first conductivity cell and corrects the boron measurement made at the second conductivity cell for the Li conductivity contribution. There are a number of differences between the Yabe '259 patent and the present invention.
Yabe '259 describes an invention that continuously blends mannitol solution with a boron-containing sample (or boron standard). This method of polyol addition is also sensitive to variations between the flow rates of the sample and mannitol streams, and therefore introduces measurements errors.
The Yabe '259 process consumes large amounts of mannitol because of the continuous addition method used. By contrast, the preferred method of polyol addition for the present invention is injection of small aliquots.
Yabe '259 does not account for the necessity of correcting for added conductivity attributable to a polyol/boron-free sample for accurate low level boron measurements. This will introduce unacceptable errors in measurements at low boron concentrations.
The present invention compares the conductivity of a polyol/boron-containing sample with that of a polyol/boron-free sample. Yabe '259 does not teach this critical step for boron measurement.
Finally, Yabe '259 does not teach deionization of polyol solution before its addition to the water sample to remove ions that could contaminate the conductivity of the polyol, thereby again introducing inconsistencies and inaccuracies into boron measurements.
A Russian journal publication entitled “Flow-Injection Determination of Boric Acid”; O. V. Krokhim, et al., Zhumal Analiticheskoi Khimii, Vol. 47, No. 5, pp. 773-775 (May 1992) describes use of a flow injection method to measure high levels of boron in water. The range of boron concentrations that can be measured linearly using this approach is reported to be from 10 ppm B to 16,000 ppm B.
Such method has some superficial similarities to the present invention in that both use a flow injection method for determining concentrations of boron, and both also use, conductivity of a polyol/boron-containing sample water solution as a measure of the amount of boron in solution.
There are a number of critical differences, however, between the method taught by the Russian journal article and the present invention. Because the method described by Krokhim et al. does not measure polyo/boron-free background, it cannot accurately measure boron at very low levels of from 0.01 ppb to 1000 ppb (1 ppm). Furthermore, the method of Krokhim et al. describes a generally linear response range from 10 ppm B to 16,000 ppm B. By contrast, the invention of the present application has a mathematically-correlated response range that starts below 0.05 ppb B and continues up to 1000 ppb B and higher.
The method as practiced by Krokhim et al. consumes large amounts of polyol by injecting a sample into a deionized (DI) water stream and then mixing this resultant stream with another stream of concentrated polyol solution. By contrast, the present invention utilizes small injections of concentrated polyol directly into the deionized water sample, thereby consuming only a small amount of polyol to provide boron concentration measurements. Such improved method, using small amounts of polyol, makes this technique compatible with on-line and compact measurement equipment, something that was virtually impossible to achieve with prior art boron measurement processes. The improved efficiency of reagent consumption with the present invention also leads to lower operating costs and smaller instrument volumes and smaller foot prints in industrial production environments.
The method taught by Krokhim et al. results in a high sensitivity to flow ratio variations of the two main streams. By contrast, the present invention does not have this problem because concentrated polyol solution is injected in very small aliquots into a sample. The mixing ratio is accurately fixed and therefore is independent of the sample flow rate.
Krokhim et al. also do not teach deionizing polyol solution to minimize conductivity background variations which will otherwise lead to measurement inconsistencies and inaccuracies.
Furthermore, Krokhim et al. do not compare the conductivity of an aqueous solution of polyol/boron-containing sample with the conductivity of a polyol/boron-free sample, a step which is critical to the present invention.
Unexamined Japanese patent application specification J 10-62371 (published Mar. 6, 1998) (“J 10-62371” hereinafter) for “A process and apparatus for measuring boron, ultrapure water production apparatus and a process for the operation thereof” teaches measurement of boron in semiconductor-quality ultrapure water using boron-polyol complex chemistry. Polyol solution in J 10-62371 is deionized to remove ions that might otherwise contaminate the polyol with conductive ions. The process adds a low concentration of polyol to a sample stream and measures the conductivity of the resulting solution at least after such addition. In a first version (FIG. 1 of J 10-62371) polyol is pumped out of a storage container, through a pump, through a mixed bed ion exchange resin and into the sample stream. The reaction product is measured with a conductivity sensor. In a second variation (FIG. 2), the conductivity of the sample stream is measured before and after injection of deionized polyol solution. In a third version (FIG. 3), polyol is continuously recirculated through a pump, a mixed bed ion exchanger, and a polyol storage tank. On command from an electronic control unit, a second pump and valve are activated to remove deionized polyol from the recirculating loop and add it to the sample stream.
This approach also has some superficial similarities to the present invention in that the detector of J 10-62371 is intended for measurement of very low levels of boron as needed in the semiconductor industry, and polyol is deionized before it is added to the sample stream. There are a number of critical differences, however, between the apparatus and method taught by such application and the present invention.
First, although the detector of J 10-62371 is intended to measure low levels of boron in ultrapure water, the design and operation of the device do not lead to sensitive enough results for practical on-line analysis of very low boron levels due to multiple, critical deficiencies in the design and operation of such detector.
One such deficiency is that the injection of polyol in J 10-62371 is continuous and not a plug injection into a tube of small internal diameter (as in the present invention), and is therefore subject to changes in accuracy of boron measurements due to flow rate variations. A second deficiency is that J 10-62371 does not teach or suggest the critical importance of routinely correcting conductivity readings for conductivity of a polyol/boron-free sample. A third deficiency is that J 10-62371 does not teach or suggest the need for using concentrated polyol solutions to obtain the critically necessary sensitivity. Because of the foregoing deficiencies, the method and apparatus of J 10-62371 cannot successfully and accurately measure boron concentrations at the very low levels of the present invention.
Thus, in J 10-62371, the conductivity of a polyol/boron-free sample is neither routinely measured nor routinely used to correct the boron response of a polyol/boron-containing sample. Instead, the approach of J 10-62371 includes the steps of measuring the conductivity of a polyol-free sample containing boron, and then subtracting this polyol-free conductivity from the conductivity of a polyol/boron-containing sample as conductivity correction. Such conductivity correction may be easier to carry out, but, critically, it yields very different and less accurate results as compared with the conductivity correction procedure of the present invention. That is because the conductivity correction method of the present invention corrects for conductivity effects attributable to polyol/boron-free sample water solution, while the J 10-62371 method does not do so. Use of relatively much higher concentration polyol solutions, another feature of the present invention as discussed below, leads to even greater conductivity effects attributable to polyol/boron-free samples thereby further increasing the need for the conductivity correction method of this invention.
In the process of J 10-62371, concentrations of polyol after mixing with boron containing samples are so low they do not exert significant conductivity increases. The concentration of mannitol solution at the conductivity cell in J 10-62371 is reported as 0.0060 moles mannitol/l. At such concentration, conductivity background from a mannitol/boron-free sample (although not even suggested by J 10-62371) is not measurable. This fact explains why Table 1 in J 10-62371 shows the same resistivity (18.2 megaohm cm) before and after mannitol is added (18.2 megaohm cm). By contrast, with the present invention, the concentration of mannitol in the mannitol/boron-free sample is 0.32 moles/l. At this fifty times higher concentration of polyol, the conductivity background is 5.1 μS/cm higher than the conductivity of ultrapure water without polyol (0.055 μS/cm), and therefore must be accounted for, which is not taught or suggested by J 10-62371. In exchange for the higher polyol concentration of the present invention necessitating an additional corrective step, however, the conductivity response per ppb of boron of the present invention is greatly increased (by 8 times at 0.05 ppb boron to 14 times at 10 ppb boron) relative to the response obtained using the technique of J 10-62371.
The concentration of polyol solution is so low in J 10-62371 that it causes measurement of boron to not be very sensitive. Thus, in such application, polyol is added at a very dilute concentration to conserve polyol use. FIG. 13 of the present application shows the critical effect of using low concentrations of polyol on measurement of boron, comparing data for the device of J 10-62371 with comparable data using the apparatus and method of the present invention. It can be seen that the conductivity response curve for data resulting from the J 10-62371 device/process is relatively flat, making it critically difficult to determine very low concentrations of boron over the range 0-10 ppb. By contrast, the conductivity response curve for comparable data generated using the present invention is quite steep thereby clearly differentiating boron concentration differences of as little as 0.05 ppb based on critically significant changes in the conductivity. Achieving such critically greater sensitivity in conductivity measurements, however, has been found to require use of much more concentrated polyol solutions, which as discussed above is completely inconsistent with the design and operation of the device taught by J 10-62371.
J 10-62371 does not teach that the difference between conductivity of a polyol/boron-containing sample and conductivity of a polyol/boron-free sample is mathematically accurately correlated to a low-level concentration of boron in such sample over the range of very low levels of boron. J 10-62371 also does not teach that the concentration of the polyol solution must be relatively high (i.e., preferably greater than 0.05 M polyol/l) in order to obtain the critical high sensitivity response needed for accurate low-level boron measurements. J 10-62371 certainly does not teach the use of both of these techniques in combination.
Another distinction between J 10-62371 and the present invention is that the former does not teach or suggest injection of microliter volumes of high concentration polyol into a boron-containing sample. Furthermore such patent application teaches continuously injecting polyol into a continuously flowing sample stream, but this approach requires a large volume of polyol reagent. Additionally, the method of J 10-62371 is sensitive to any changes in the ratio of flow rate of polyol injection relative to sample flow rate. By contrast, the present invention is not sensitive to changes in sample flow rate because in the preferred embodiment a small plug of polyol is inserted into the sample stream, and dilution is fixed by a combination of the laminar flow, surface tension and diffusion within the micro dimensions of the apparatus.
Still another critical difference is that the present invention uses just one pump and one valve both to recirculate polyol solution through the deionization resin bed and to insert a micro-plug of polyol into a tube containing the sample stream and having a small inside diameter. By comparison, FIG. 3 of J 10-62371 requires two pumps to achieve the same functions. Where J 10-62371 teaches use of only one pump (as shown in FIGS. 1 and 2 thereof), it does not include recirculation of polyol through a deionization resin bed. As shown in FIGS. 1 and 2 of said application, a pump pumps polyol from polyol storage compartment, through the pump, through a deionization module and then into a sample stream. As a result, the methods of FIGS. 1 and 2 of J 10-62371 require accurate flow measurements and controls, and use a large amount of polyol for each boron analysis.
The consumption of polyol is much less for the present invention than for the J 10-62371 because in the present invention polyol micro-pulses are injected into samples. By contrast, the Japanese application uses a continuously flowing stream of dilute polyol added to continuously flowing sample, even when polyol addition is only made periodically. By contrast, the present invention adds polyol accurately in micro-injections of only 25 μl or 50 μl. The apparatus of J 10-62371 requires large tanks to store the large volume of polyol required for such process. Typical flow rates of polyol and sample used in J 10-62371 are 1.44 /hr (24 ml/min.) and 15 l/hr (250 ml/min.) respectively. The mannitol has a concentration of 12.5 grams/liter or 12.5/182=0.069 moles/l. Therefore, the method of J 10-62371 uses 0.069 M/l×1.44 l/hr.=0.01 moles (18 grams) mannitol/hr. to measure boron in the sample. By comparison, the present invention uses only 50 μl of a 1.0 molar (182 g/l) mannitol solution or 5×10−5 moles (0.009 grams) mannitol/injection. Based on a typical 12 injections per hour, the present invention uses 6×10−4 moles (0.1 grams) mannitol per hour, only about {fraction (1/180)}th the mannitol consumption of J 10-62371.
Another recent journal article entitled “Determination of Boron by Flow Injection Analysis Using a Conductivity Detector” by S. D. Kumar, et al.; Analytical Chemistry, Vol. 71, No. 13 (Jul. 1, 1999) pages 2551 to 2553, teaches use of an injection valve to inject a 100 μl volume of a boron containing sample into a stream containing 0.3 M mannitol flowing at 1 ml/minute. The combined streams are flowed into a mixing tube, and then into a 6 μl volume conductivity cell where conductivity change is measured. The linear boron measurement range of this method is reported as 0-20 ppm as B, and the lower limit of detection is reported as 10 ppb as B. When there are interfering ions in the sample, a pre-treatment procedure to remove them is employed. This pre-treatment procedure, however, requires stirring a strong base anion exchange resin in the Cl− form with an aliquot of the sample to convert all ionized anions to the Cl− form. After filtration, such pre-treated solution is passed through a column containing cation exchange resin in the Ag+ form to remove all chloride and ionized cations. As Kumar et al. point out, the method does not remove weak acid anions such as acetate, formate and bicarbonate quantitatively.
Here again, the apparatus and method of Kumar et al. have some superficial similarities to the present invention in that both methods and apparatus use flow injection analysis to measure boron in solution, and the concentration of the mannitol solution is similar in both designs. Kumar et al. inject a boron-containing sample into a flowing mannitol solution whereas the present invention, critically, injects concentrated mannitol into a flowing sample stream.
There are numerous critical differences between the method and apparatus taught by Kumar et al. and the present invention. The dynamic range of the analyzer of the present invention is from 0-1000 ppb as B, whereas the device of Kumar et al. measures from 0-20 ppm as B. For the Kumar et al. device, the lower limit of detection (LOD) is 10 ppb as B. while the present invention has a LOD as low as 0.05 ppb as B (200 times lower). The response factor is also critically different between the two methods. The Kumar et al. response is 0.5 μS/ppm B. By contrast, the present invention shows a response factor of 17 μS/ppm B, a 34 times improvement.
Still another critical deficiency is that the Kumar et al. method of injecting a sample as a slug into a flowing 0.3 M mannitol stream is not efficient with respect to reagent consumption. If a measurement were made every 10 minutes, the amount of polyol consumed over 6 months would be 13,100 ml of 0.3 M mannitol (720 grams mannitol) calculated as (0.5 ml/measurement)×6 measurements/hr.×24 hr./day×182.5 days). In contrast to such method, the method of the present invention provides conversely and critically for the injection of polyol as a micro slug directly into a flowing sample stream (both for boron-containing water and for boron-free water). The invention of the present application requires an injection volume of 25 μL of polyol per measurement into the sample stream. The volume of 1 M mannitol used over the same six month period thus would be only 655 ml (120 grams mannitol). This amount is six times less mannitol than that required for the process of Kumar et al., and twenty times less solution. This extremely efficient use of polyol is very important and practical for on-line boron measurements as it makes possible the use of reagent containers of a reasonable size.
Kumar et al. do not teach deionization of concentrated mannitol solution. This step has been found to be critically important if low-level boron concentrations are to be measured accurately. Such failure by Kumar et al. probably explains their LOD of only 10 ppb as B.
The present invention uses a micro conductivity cell that is only one-third the volume of the Dionex conductivity cell used in the Kumar et al. device. Both cells have a cell constant of one. The smaller conductivity detector volume of the present invention improves the conductivity peak resolution, accuracy and sensitivity. Kumar et al. also do not teach the importance of thermal correction of conductivity measurements, nor do they teach the importance of demineralizing polyol.
Thus, there remains an unmet need in the art for an inexpensive boron detection and measurement system which can be made light-weight, compact and portable, and which is capable of accurately and reproducibly measuring extremely low concentrations of boron in water on-line. This need is met, and the aforementioned drawbacks and limitations of the prior art boron detectors are overcome, in whole or in part, with the low-level boron detection and measurement system of the present invention.